Conducting reactions in Leidenfrost-levitated droplets

ABSTRACT

The invention generally relates to conducting reactions in Leidenfrost-levitated droplets.

RELATED APPLICATIONS

The present application claims the benefit of and priority to U.S.provisional application Ser. No. 62/346,594, filed Jun. 7, 2016, andU.S. provisional application Ser. No. 62/346,213, filed Jun. 6, 2016,the content of each of which is incorporated by reference herein in itsentirety.

GOVERNMENT INTEREST

This invention was made with government support under DE-FG02-06ER15807awarded by the U.S. Department of Energy and CHE-1307264 awarded by theNational Science Foundation. The government has certain rights in theinvention.

FIELD OF THE INVENTION

The invention generally relates to conducting reactions inLeidenfrost-levitated droplets.

BACKGROUND

Electrospray ionization (ESI) is a commonly used ionization techniquefor mass spectrometry (MS) in which analyte ions are generated andeffectively transported from solution phase to the gas phase. In ESI,analytes in a charged solution are sprayed pneumatically to produce aplume of charged microdroplets. These microdroplets undergo desolvationand coulombic fission events to produce numerous smaller and moreconcentrated droplets. These microdroplets are the nexus for acceleratedchemical reactions.

On-line monitoring experiments have demonstrated that increasing adistance between the sprayer and a mass spectrometer inlet increases aproduct to starting material ratio. It has been demonstrated thatreaction rate acceleration in spray (compared to bulk) for thePomeranz-Fritsch synthesis of isoquinoline was greater than six ordersof magnitude. In a study of the Claisen-Schmidt base-catalyzedcondensation, it was shown that acceleration is not simply a result ofincreased concentration dependence but is strongly influenced byinterfacial effects. Successful efforts at milligram scale chemicalsynthesis using reactions in electrosprayed droplets have been reported.

SUMMARY

The invention provides a new approach for accelerating chemicalreactions. Aspects of the invention take advantage of the Leidenfrosteffect. The Leidenfrost effect occurs when a liquid is brought to asurface heated to a temperature significantly higher than the liquid'sboiling point. As the droplet approaches the super-heated surface, aninsulating vapor layer is formed that keeps the solution from boilingrapidly. With this vapor cushion in place, the droplet levitates on thesurface and solvent gradually evaporates. The methods of the inventionuse the Leidenfrost effect to perform accelerated chemical reactions indroplets. In certain embodiments, the reaction is conducted whilemaintaining a substantially constant volume of the droplets. Bymaintaining Leidenfrost droplets at constant volume is it shown thatacceleration is mainly due to interfacial effects. In other embodiments,the droplet reduces in volume as the reaction occurs and the reactionproduct is formed in a Leidenfrost droplet having a reduced volume. Theability to multiplex Leidenfrost microreactors and to use them tosynthesize milligram quantities of reaction product is alsodemonstrated.

In certain aspects, the invention provides methods for forming areaction product that involve conducting a reaction within aLeidenfrost-levitated droplet while maintaining a substantially constantvolume of the Leidenfrost-levitated droplet, thereby forming a reactionproduct within the Leidenfrost-levitated droplet. Maintaining thesubstantially constant volume may involve introducing droplets of puresolvent or the reaction mixture to the Leidenfrost-levitated droplet. Arate at which the droplets of pure solvent or the reaction mixture areintroduced to the Leidenfrost-levitated droplet may depend on anevaporation rate of the Leidenfrost-levitated droplet. In certainembodiments, the methods are conducted without the use of surfactants.

The methods of the invention may further involve conducting multiplereactions in multiple separate Leidenfrost-levitated droplets. Incertain embodiments, the multiple separate Leidenfrost-levitateddroplets are merged with each other. The multiple reactions may be thesame. Alternatively, the multiple reactions may be different.

Other aspects of the invention provide methods for forming a reactionproduct that involve introducing a first liquid droplet onto a heatedsurface that is at a temperate that is at or above a Leidenfrost pointof the first liquid such that the first liquid droplet levitates on theheated surface. The first liquid droplet includes reagents for areaction. A reaction is conducted in the first liquid droplet using thereagents in order to form a reaction product. The methods then involveintroducing one or more subsequent liquid droplets to the first liquiddroplet on the heated surface such that the one or more subsequentliquid droplets merge with the first liquid droplet. The one or moresubsequent liquid droplets may include the reagents for the reaction orpure solvent or another type of liquid. In some embodiments thecomposition of the gas surrounding the Leidenfrost droplet may beadjusted so as to produce desired reactions.

In certain embodiments, the first liquid droplet is maintained at asubstantially constant volume throughout the method. A rate at which theone or more subsequent liquid droplets are introduced to the firstliquid droplet will depend on an evaporation rate of the first liquiddroplet. In certain embodiments, the method is conducted without the useof surfactants.

The methods of the invention may further involve analyzing the reactionproduct. Analyzing may be by any method known in the art. An exemplarymethod for analysis uses a mass spectrometry technique.

Other aspects of the invention provide methods for forming a reactionproduct that involve conducting a reaction within aLeidenfrost-levitated droplet, thereby forming a reaction product withinthe Leidenfrost-levitated droplet. In certain embodiments, the dropletreduces in volume as the reaction occurs.

For the embodiments described above, a rate of the reaction may beaccelerated as compared to a rate of the reaction conducted outside ofthe Leidenfrost-levitated droplet. The methods of the invention havemany uses, and one exemplary use is performing degradation studies ofcommercial products like lubricants, cosmetics, foodstuffs, medicines,etc., in which a rate of the reaction, i.e. degradation, is acceleratedwithin the Leidenfrost-levitated droplet as compared to a rate of thereaction conducted outside of the Leidenfrost-levitated droplet. Themethods of the invention significantly cuts time for such studies.Additionally, the methods of the invention allow for multiplexing, whichmakes the methods especially useful in these sorts of product spoilagestudies.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a Leidenfrost reaction system used in methods of theinvention in which additional reaction mixture is added via pipette tomaintain constant volume Leidenfrost droplets. The added droplets impactthe surface and fuse with the main levitated droplet which sits atop acushion of solvent vapor.

FIGS. 2A-B are spectra showing that bulk-phase reaction of 4 and 5 after2 min showed mainly reagent 5 and some intermediate (6) while theparallel Leidenfrost experiment provided significant product (7)formation.

FIGS. 3A-B are spectra showing bulk-phase Katritsky reaction with andwithout base. FIGS. 3A-B show less product (10) vs. starting material(8) than do the corresponding Leidenfrost experiments in FIGS. 3C-D,which display acceleration factors of 50 and 8, respectively.

FIGS. 4A-D show surface effects in Claisen-Schmidt reactions. FIG. 4Ashows 2 min bulk phase reaction showing 13/11 ratio. FIG. 4B showsLeidenfrost reaction showing an acceleration factor of 17. Addition oftriton x-100 at 1% and 5% v/v suppresses the acceleration effect to afactor of just 2 and units as shown in FIGS. 4C-D, respectively. Ionswith spacing of 44 are PEG spaced peaks of triton.

FIGS. 5A-D show Bulk-phase reaction without (FIG. 5A) and withadditional acid (FIG. 5B) at 2 min reaction time shows significantlyless reaction product than do the corresponding 2 min Leidenfrostdroplet experiments (FIGS. 5C-D). Note that reagent appears as theprotonated and the sodiated species (m/z 362 and m/z 384) and their sumwas used to calculate the acceleration factor. Acid catalysis isresponsible for the difference between FIGS. 5A and C and FIGS. 5B andD.

FIG. 6 shows photographs of a multiplexing approach where the separatedLeidenfrost droplets are continually replenished with reaction mixture(left) and when the reaction had run for ca. 2 mins and all but two ofthe droplet spacers have been removed allowing the droplets to fuseprior to collection (right).

FIG. 7A shows bulk-phase at 4× concentration after 2 mins. FIG. 7B showsbulk-phase at standard concentrations. FIG. 7C shows Leidenfrost dropletwith solvent added and not the traditional reaction mixture addition.FIG. 7D shows the Leidenfrost droplet with reaction mixture addition.

DETAILED DESCRIPTION

Leidenfrost-levitated droplets can be used to accelerate chemicalreactions in processes that appear to be similar to reactionacceleration in charged microdroplets produced by electrosprayionization. Reaction acceleration in Leidenfrost droplets has beendemonstrated for base-catalyzed Claisen-Schmidt condensations, hydrazoneformation from pre-charged and neutral ketones and for Katritzkypyrylium to pyridinium conversion under various reaction conditions.Comparisons with bulk reactions and with reactions in electrosprayedmicrodroplets show intermediate values of acceleration factors (2-50).In certain embodiments, by maintaining Leidenfrost droplets at constantvolume, it is shown that acceleration is mainly due to interfacialeffects. Accelerated reactions are also conducted in which the volume ofthe Leidenfrost-levitated droplet is allowed to decrease. The ability tomultiplex Leidenfrost microreactors and to use them to synthesizemilligram quantities of reaction product is demonstrated.

In certain embodiments, Leidenfrost droplets were maintained at constantvolume (ca. 0.5 mL) by adding approximately 2 mL of the reaction mixtureover the course of droplet levitation (2 min) (FIG. 1). Because thedroplets concentrate the reaction mixture by approximately 4 times overthe initial concentration during the constant volume experiment,bulk-phase reactions were also performed at 4 fold higher concentrationin order to correct for the concentration effect. Additional experimentswere performed by maintaining constant volume with pure solvent andcomparative bulk reactions were performed at room temperature at theinitial concentrations used for the Leidenfrost experiments. Theseadditional experiments helped separate effects of concentration changesfrom those associated with other features of the levitated droplets,notably interfacial effects.

Leidenfrost experiments were conducted for 2 min in a petri dish atop aheater/stirrer (Fisher Scientific) at a surface temperature between 400and 540° C. All analyses were performed by nanoESI using an LTQXL-Orbitrap hybrid mass spectrometer (Fisher Scientific, San Jose,Calif.) under conditions shown not to lead to reaction acceleration inthe analysis step. Additional details on each reaction system studiedcan be found in Example 1 below. All reagents and solvents werepurchased from Sigma-Aldrich (St. Louis, Mo.) unless otherwisespecified. Discussion of the determination of the acceleration factorand the nESI-MS method of chemical analysis can be found in Examples 2and 3 below.

Hydrazone formation between cortisone (1) and Girard's Reagent T (2) toform the pre-charged hydrazone product (3) (Scheme 1) was selected as amodel reaction because a similar system had been characterized in thebulk-phase and using acceleration in desorption electrospray ionization(DESI) droplets. The results (Table 1) show significant reactionacceleration, expressed as an acceleration factor and defined as theratio of the MS product ion signal to the reagent ion signal in theLeidenfrost vs. the ratio in the bulk experiment. Acceleration factorsmeasuring the product/reagent ratios with the 5 mM bulk concentrationswould be 17; however, the concentration increase in the Leidenfrostdroplets due to evaporation must be taken into account. This was done bymeasuring the bulk reaction data measured at four times the initialconcentration. Hydrazone formation from the Leidenfrost droplets thenshows a product (3)/reagent (1) ion ratio which is notably higher(factor of 4) than that for the bulk solution (Example 4 below). Notethat addition of acid increases product formation in both experiments.Note that in this experiment and those with other reactions, the productsignals in Leidenfrost droplets after two minutes matched those reachedafter ca. 1 or more hours in the corresponding bulk experiment (Table1).

TABLE 1 Bulk-phase and Leidenfrost droplet reactions and accelerationfactors (acc. factor) Girard T/Hydrazine Reaction Products 3/1Phenylhydrazine/Isatin Reaction Products 7/5 Ratio of ion intensitiesRatio of ion intensities LF Bulk, Reaction x4 Droplet, x1 Reaction timeadd’l LF X1 add’l add’l time x4 LF x1 (min) x4 acid Droplet Bulk acidacid (min) conc. Droplet Bulk  2 0.1 0.5 0.4 0.05 1.0 0.06  2  0.05 0.30.03  60 0.6 2.0 Acc. factor: 4 Acc. factor: 2  60 0.1 Acc. factor: 6120 1.0 2.8 120 0.2 180 3.2 3.4 180 0.3 Katritsky reaction products 10/8Claisen-Schmidt Reaction Products 13/11 Ratio of ion intensities Ratioof ion intensities x4 LF x1 Reaction conc. Droplet Bulk Reaction time x4with LF x1 with with time x4 LF x1 (min) conc. base Droplet Bulk basebase (min) conc. Droplet Bulk  2 0.1 0.4 0.8 0.05 20   0.2   2 0.6 10  0.25  60 0.2 0.8 Acc. factor: 8 Acc. factor: 50  60 3.3 Acc. factor: 17120 0.5 2.1 120 10   180 0.6 10   180 18  

Reaction of neutral isatin (4) with phenylhydrazine (5) to form thehydrazone product (7) via a detectable pre-condensation intermediate (6)(Scheme 2) was investigated. This reaction is known to be accelerated inESI when increasing the distance between the spray source and MS inlet,ascribed to solvent evaporation and increased concentrations andsurface/volume ratio. The Leidenfrost version of this reaction alsoshowed a substantial acceleration factor (×6 after correction forconcentration effects) over the bulk phase reaction. The data are shownin Table 1 and in FIGS. 2A-B.

The Katritzky reaction (Scheme 3) between 2,4,6-triphenylpyrylium (8)and 4-methoxyanaline (9) to yield the corresponding pyridinium cation(10) was investigated also. This base-catalyzed reaction again showed astrong dependence on the catalyst. The reaction had been previouslyinvestigated by paper spray ionization (thin film experiment) and itshowed a significant reaction acceleration. Reaction (2 min, constantvolume) in Leidenfrost droplets showed acceleration factors of 8 and 50for the reaction performed without and with the base catalyst,respectively (FIGS. 3A-D, Table 1). Note that the time taken for thebulk reaction to reach the same product/reagent ion intensities is analternative measure of acceleration factor. In this case it is more than11 hour vs. 2 min.

The potential for scaling-up the Leidenfrost reactor was explored usingthe Katritsky reaction. In a prototype experiment, eight ceramic spacerswere used to isolate eight droplets and keep them from fusing. Thedroplets were levitated for ca. 2 minutes while their individual volumes(0.5 mL) were maintained by addition of fresh reaction mixture. When thespacers were removed to allow the droplets to fuse, a single largedroplet formed and it was immediately extracted from the surface. Theresulting sample 18 mg, was analyzed by MS and found by standardaddition of reagent 8 to be 90% pure (10% unconverted 8). Thisexperiment illustrates the ease of manipulating these droplets, theability to multiplex this meso-scale reactor system and the synthesisorganic compounds on the mg scale. Example 5 below contains details ofthe multiplexing approach.

Lastly, the Claisen-Schmidt base-catalyzed condensation was investigatedbecause the acceleration of this reaction has been well-characterized bypaper spray and ESI. 6-Hydroxy-1-indanone (11) was reacted withbenzaldehyde (12) in the presence of base to form the condensationproduct (13). Both the starting material (11) and the product (13)presumably deprotonate at the aromatic hydroxyl and have similarionization efficiencies in the negative ion mode. The mass spectra(FIGS. 4A-D) show significantly more product for the Leidenfrostexperiment than for the bulk-phase reaction (17× acceleration factor).

In order to explore the role of the surface on Leidenfrost dropletacceleration, a surfactant was added to reaction mixture. Addition ofTriton-x-100:

in varied amounts showed significant effects on the reactionacceleration. Addition of 0.01% (v/v) triton showed no effect but 1%(v/v) triton containing reaction mixture, when subject to the sameLeidenfrost conditions, showed significant suppression of acceleration(FIGS. 4A-D) while 5% (v/v) triton suppressed the reaction to bulkvalues. Higher concentrations of triton would not allow the droplet tobe maintained at the Leidenfrost temperature. This experimentdemonstrates the role of the surface in the acceleration of thesereactions since small concentrations of triton block the surface anddiminish accelerated product formation. Further details on the roles ofconcentration and Leidenfrost methods can be found in Example 6 below.

Acceleration of a variety of chemical reactions in Leidenfrost dropletshas been demonstrated. By controlled additions of reaction mixture tothe droplet and comparison bulk-phase reactions the accelerationphenomenon is shown to be due mainly to interfacial reactivity, althoughconcentration effects do play a role as detailed in in the Examples.Surfactant addition experiments confirm the role of surface activity inthe accelerated reactions which in this respect are similar to thereaction acceleration seen in spray-based microdroplets. The methods ofthe invention may be used to screen reactions and create milligramquantities of material in minutes.

As mentioned herein, analyzing the reaction product can be by massspectrometry, however, any analytical technique, such as liquidchromatography, UV analysis, ELSD analysis, optical spectroscopy, etc.,can be used. The following sections discuss analysis by massspectrometry.

Ion Traps and Mass Spectrometers

Any mass spectrometer (e.g., bench-top mass spectrometer of miniaturemass spectrometer) may be used to analyze reaction products from methodsof the invention and in certain embodiments the mass spectrometer is aminiature mass spectrometer. An exemplary miniature mass spectrometer isdescribed, for example in Gao et al. (Anal. Chem. 2008, 80, 7198-7205.),the content of which is incorporated by reference herein in itsentirety. In comparison with the pumping system used for lab-scaleinstruments with thousands of watts of power, miniature massspectrometers generally have smaller pumping systems, such as a 18 Wpumping system with only a 5 L/min (0.3 m³/hr) diaphragm pump and a 11L/s turbo pump for the system described in Gao et al. Other exemplaryminiature mass spectrometers are described for example in Gao et al.(Anal. Chem., 2008, 80, 7198-7205.), Hou et al. (Anal. Chem., 2011, 83,1857-1861.), and Sokol et al. (Int. J. Mass Spectrom., 2011, 306,187-195), the content of each of which is incorporated herein byreference in its entirety.

In certain embodiments, a Discontinuous Atmospheric Pressure Interface(DAPI) can be utilized with the mass spectrometer. A DAPI isparticularly useful when coupled to a miniature mass spectrometer, butcan also be used with a standard bench-top mass spectrometer.Discontinuous atmospheric interfaces are described in Ouyang et al.(U.S. Pat. No. 8,304,718 and PCT application number PCT/US2008/065245),the content of each of which is incorporated by reference herein in itsentirety.

Any ion trap known in the art can be used in systems of the invention.Exemplary ion traps include a hyperbolic ion trap (e.g., U.S. Pat. No.5,644,131, the content of which is incorporated by reference herein inits entirety), a cylindrical ion trap (e.g., Bonner et al.,International Journal of Mass Spectrometry and Ion Physics,24(3):255-269, 1977, the content of which is incorporated by referenceherein in its entirety), a linear ion trap (Hagar, Rapid Communicationsin Mass Spectrometry, 16(6):512-526, 2002, the content of which isincorporated by reference herein in its entirety), and a rectilinear iontrap (U.S. Pat. No. 6,838,666, the content of which is incorporated byreference herein in its entirety).

Ionization Sources

For mass spectrometry analysis, the reaction product is typically firstionized. Any type of ionizing source known in the art may be used, andthe source used will depend, at least in part, on the reaction productto be analyzed. Exemplary mass spectrometry techniques that utilizeionization sources at atmospheric pressure for mass spectrometry includepaper spray ionization (ionization using wetted porous material, Ouyanget al., U.S. patent application publication number 2012/0119079),electrospray ionization (ESI; Fenn et al., Science, 1989, 246, 64-71;and Yamashita et al., J. Phys. Chem., 1984, 88, 4451-4459.); atmosphericpressure ionization (APCI; Carroll et al., Anal. Chem. 1975, 47,2369-2373); and atmospheric pressure matrix assisted laser desorptionionization (AP-MALDI; Laiko et al. Anal. Chem., 2000, 72, 652-657; andTanaka et al. Rapid Commun. Mass Spectrom., 1988, 2, 151-153,). Thecontent of each of these references is incorporated by reference hereinin its entirety.

Exemplary mass spectrometry techniques that utilize direct ambientionization/sampling methods include desorption electrospray ionization(DESI; Takats et al., Science, 2004, 306, 471-473, and U.S. Pat. No.7,335,897); direct analysis in real time (DART; Cody et al., Anal.Chem., 2005, 77, 2297-2302.); atmospheric pressure dielectric barrierdischarge Ionization (DBDI; Kogelschatz, Plasma Chemistry and PlasmaProcessing, 2003, 23, 1-46, and PCT international publication number WO2009/102766), and electrospray-assisted laser desorption/ionization(ELDI; Shiea et al., J. Rapid Communications in Mass Spectrometry, 2005,19, 3701-3704.). The content of each of these references in incorporatedby reference herein its entirety.

On-Line Reaction Monitoring

In certain embodiments, monitoring of the reaction, in real-time, may bedesirable. Accordingly, certain embodiments utilize on-line reactionmonitoring systems for monitoring the formation of a reaction productwithin a Leidenfrost-levitated droplet. The volume of the droplet ismaintained as discussed above and the rate/amount of addition of solventor reagents is determined by the rate and/or amount of liquid removedfrom the Leidenfrost-levitated droplet in which the reaction isoccurring. The removed portion of liquid is directly transferred to theon-line reaction monitoring system. The system may be equipped with atransfer line that couples to the to the Leidenfrost-levitated dropletin which the reaction is occurring.

An exemplary on-line reaction monitoring system is described for examplein Cooks et al. (U.S. Pat. No. 9,500,623), the content of which isincorporated by reference herein in its entirety. Briefly, a transferline is positioned in proximity to the Leidenfrost-levitated droplet inwhich the reaction is occurring or the reaction in theLeidenfrost-levitated droplet is conducted inside a pressurized vesselto which the transfer line is connected. A portion of theLeidenfrost-levitated droplet in which the reaction is occurring istransferred through the transfer line to an emitter-spray tip.Ionization may be produced by inductive electrospray ionization. Apositive electrospray potential applied to an electrode near to but notin physical contact with the solution being sprayed or the emitter ispulsed repeatedly to produce strong electric fields of both polaritieswhich result in bursts of charged droplets being emitted from thesolution while avoiding direct physical contact between the high voltageand the reaction solution. Sheath gas may be used to help in thenebulization process and minimize size-variation in the droplets. Samplesplitting was useful in accelerating the flow rate of sampling anddecreasing the delay time while avoiding contamination of the MS inlet.Mass spectra recorded as a function of time give the desired kineticinformation.

Ion Transfer

In certain embodiments, ionization of a reaction product within aLeidenfrost-levitated droplet may be desirable at the location where theLeidenfrost-levitated droplet resides. In such embodiments, thegenerated reaction product ions may be transferred using an ion transferline to an analysis system, such as a mass spectrometer. Systems andmethods of transferring ions are described, for example in Ouyang et al.(U.S. Pat. No. 8,410,431), the content of which is incorporated byreference herein in its entirety. Such devices generate a laminar gasflow that allows for efficient transfer of ions without significant lossof signal intensity over longer distances, such as distances of at leastabout 5 cm, at least about 10 cm, at least about 20 cm, at least about50 cm, at least about 100 cm, at least about 500 cm, at least about 1 m,at least about 3 m, at least about 5 m, at least about 10 m, and otherdistances. Ion transfer devices of the invention are useful for chemicalanalysis in situations in which it is important for the ion focusingdevice or instrument and the object being examined to be in differentlocations. Generally, the ion transfer member is operably coupled to agas flow generating device, in which the gas flow generating deviceproduces a laminar gas flow that transfers the gas phase ions to aninlet of the ion focusing device.

Ion transfer devices of the invention provide enlarged flow to carryions from a distant sample to the ion focusing device. The basicprinciple used in the transport device is the use of the gas flow todirect gas and ions into the ion transfer member and to form a laminarflow inside the ion transfer member to keep the ions away from the wallswhile transferring the gas and ions through the ion transfer member. Theanalyte ions of interest are sampled at some point downstream along theion transfer member. The laminar flow is achieved by balancing theincoming and outgoing gas flow. Thus recirculation regions and/orturbulence are avoided. Thus, the generated laminar flow allows for highefficient ion transport over long distance or for sampling of ions overlarge areas.

Ion transfer devices of the invention also provide enlarged flow tocarry ions from the ion source to the ion focusing device. Additionalgas flow provided by a miniature sample pump connected with the iontransfer member facilitates ion transfer from an ambient ionizationsource to the vicinity of the ion focusing device.

As described in certain embodiments, an ion transfer member, e.g., atube with an inner diameter of about 10 mm or greater, may be used totransfer ions from the ionization source to the ion focusing device. Thelarger opening of the ion transfer member, as compared to the opening ofthe inlet of the ion focusing device, is helpful for collection ofsample ions generated in a large space, e.g. on a surface of large area.The large flow conductance of the ion transfer member allows the gascarrying ions to move toward the inlet of the ion analysis device at afast flow rate. The ion transfer member is coupled to a gas flowgenerating device. The gas flow generating device produces a gas flowinside the ion transfer member. The inlet of the ion analysis devicereceives the ions transferred from the ambient ionization source.

The ion transfer member may be any connector that allows for productionof a laminar flow within it and facilitates transfer of ions withoutsignificant loss of ion current. Exemplary ion transfer members includetubes, capillaries, covered channels, open channels, and others. In aparticular embodiment, the ion transfer member is a tube. The iontransfer member may be composed of rigid material, such as metal orglass, or may be composed of flexible material such as plastics,rubbers, or polymers. An exemplary flexible material is TYGON tubing.

The ion transfer member may be any shape as long the shape allows forthe production of a flow to prevent the ions from reaching the internalsurfaces of the ion transfer member where they might become neutral. Forexample, the ion transfer member may have the shape of a straight line.Alternatively, the ion transfer member may be curved or have multiplecurves.

The ion transfer member is coupled to a gas flow generating device. Thegas flow generating device is such a device capable of generating a gasflow through the ion transfer member. The gas flow generating devicefacilitates transfer of the ions from the ambient ionization source tothe inlet of the ion analysis device. In certain embodiments, the gasflow generating device is a pump with a high flow rate and a lowcompression ratio. An example of such a pump is that found in a shopvacuum or a small sample pump. The proper pumps used for the couplingare different from those used for a mass spectrometer, e.g. a rotaryvane pump or a turbo molecular pump, which pumps have a high compressionratio. The high compression ratio pumps of a mass spectrometer cannot beconnected to the atmosphere through an opening of the conductancedescribed here. For example, Cotte-Rodriguez et al. (Chem. Commun.,2006, 2968-2970) describe a set-up in which the inlet of the massspectrometer was elongated and gas flow generated by the pump inside amass spectrometer was used to transfer ions over a distance up to 1 m.The ions were transferred from the atmosphere to a region at about 1torr. A significant loss in signal occurred for the transfer of the ionsusing the set-up described in Cotte-Rodriguez, and ions generated over alarge area could not be efficiently collected into the inlet.

In other embodiments, the gas flow generating device is the ambientionization source. For example, a source used for desorptionelectrospray ionization (DESI) generates a gas flow sufficient toproduce a laminar flow through the ion transfer member, and thusproduces a laminar gas flow that transfers the gas phase ions over along distance to an inlet of the ion analysis device.

Numerous additional devices may be coupled with the ion transfer memberto further facilitate transfer of the ions from the ambient ionizationsource to the inlet of the ion focusing device. For example, an electriclens may be used to focus the ions toward the center of the ion transfermember while the gas flow generating device pumps away neutral gases. Inother embodiments, an electro-hydrodynamic lens system may beimplemented to use the air dynamic effects to focus the heavierparticles and to use the electric field to focus the charged particlestoward the center of the ion transfer member.

In other embodiments, a distal end of the ion transfer member mayinclude a plurality of inlets for transferring ions from multiplelocations to the inlet of the ion focusing device. In still otherembodiments, the ion transfer member includes additional features toprevent ions from being adsorbed onto the inside wall. For example, adielectric barrier discharge (DBD) tubing is made from a double strandedspeaker wire. The insulator of the wire serves as the dielectric barrierand the DBD occurs when high voltage AC is applied between the twostrands of the wire. The DBD inside the tube prevents the ions fromadsorbing onto the wall and provide a charge-enriched environment tokeep the ions in the gas phase. This DBD tube can also be used forionizing the gas samples while transferring the ions generated to theinlet of the ion focusing device. The DBD tube can also be used for ionreactions while transferring the ions generated to the inlet of the ionfocusing device.

Collection of Reaction Product Without or After Mass-Selective Analysis

In certain embodiments, it may be desirable to collect the reactionproduct from the Leidenfrost-levitated droplet without or aftermass-selective analysis. Systems and methods for collecting ions thathave been analyzed by a mass spectrometer are shown in Cooks, (U.S. Pat.No. 7,361,311), the content of which is incorporated by reference hereinin its entirety. In another embodiment, reaction product may becollected without mass analysis (See U.S. Pat. No. 9,184,038, thecontent of which is incorporated by reference herein in its entirety).The collected reaction product may then be subsequently analyzed by anysuitable technique, such as infrared spectrometry or mass spectrometry.

Generally, the preparation of microchips arrays of metal ions firstinvolves the ionization of the metal. The metal ions can be produced byany of the methods discussed above. The ions can then be focused andcollected using methods described below or can first be separated basedon their mass/charge ratio or their mobility or both their mass/chargeratio and mobility. For example, the ions can be accumulated in an ionstorage device such as a quadrupole ion trap (Paul trap, including thevariants known as the cylindrical ion trap and the linear ion trap) oran ion cyclotron resonance (ICR) trap. Either within this device orusing a separate mass analyzer (such as a quadrupole mass filter ormagnetic sector or time of flight), the stored ions are separated basedon mass/charge ratios. Additional separation might be based on mobilityusing ion drift devices or the two processes can be integrated. Theseparated ions are then deposited on a microchip or substrate atindividual spots or locations in accordance with their mass/charge ratioor their mobility to form a microarray.

Whether or not mass-selection is used, the microchip or substrate ismoved or scanned in the x-y directions and stopped at each spot locationfor a predetermined time to permit the deposit of a sufficient number ofmolecules to form a spot having a predetermined density. Alternatively,the gas phase ions can be directed electronically or magnetically todifferent spots on the surface of a stationary chip or substrate. Themolecules are preferably deposited on the surface with preservation oftheir structure, that is, they are soft-landed.

In embodiments in which ions are collected without prior separation, thecollection surface is operably coupled to receive the spray includingthe ions, as illustrated in FIG. 1. In embodiments that first usemass-selection, the surface is located behind the detector assembly ofthe mass spectrometer. In embodiments that use an ion focusing device,the surface for ion landing is located after the ion focusing device.

In embodiments that use mass-selection prior to ion landing, the highvoltages on the conversion dynode and the multiplier are turned on andthe ions are detected to allow the overall spectral qualities,signal-to-noise ratio and mass resolution over the full mass range to beexamined. In the ion-landing mode, the voltages on the conversion dynodeand the multiplier are turned off and the ions are allowed to passthrough the hole in the detection assembly to reach the landing surfaceof the plate (such as a gold plate). The surface is grounded and thepotential difference between the source and the surface is 0 volts.

An exemplary substrate for soft landing is a gold substrate (20 mm×50mm, International Wafer Service). This substrate may consist of a Siwafer with 5 nm chromium adhesion layer and 200 nm of polycrystallinevapor deposited gold. Before it is used for ion landing, the substrateis cleaned with a mixture of H₂SO₄ and H₂O₂ in a ratio of 2:1, washedthoroughly with deionized water and absolute ethanol, and then dried at150° C. A Teflon mask, 24 mm×71 mm with a hole of 8 mm diameter in thecenter, is used to cover the gold surface so that only a circular areawith a diameter of 8 mm on the gold surface is exposed to the ion beamfor ion soft-landing of each mass-selected ion beam. The Teflon mask isalso cleaned with 1:1 MeOH:H₂O (v/v) and dried at elevated temperaturebefore use. The surface and the mask are fixed on a holder and theexposed surface area is aligned with the center of the ion optical axis.

Any period of time may be used for landing of the ions. Inmass-selection embodiments, between each ion-landing, the instrument isvented, the Teflon mask is moved to expose a fresh surface area, and thesurface holder is relocated to align the target area with the ionoptical axis. After soft-landing, the Teflon mask is removed from thesurface.

Incorporation by Reference

References and citations to other documents, such as patents, patentapplications, patent publications, journals, books, papers, webcontents, have been made throughout this disclosure. All such documentsare hereby incorporated herein by reference in their entirety for allpurposes.

Equivalents

Various modifications of the invention and many further embodimentsthereof, in addition to those shown and described herein, will becomeapparent to those skilled in the art from the full contents of thisdocument, including references to the scientific and patent literaturecited herein. The subject matter herein contains important information,exemplification and guidance that can be adapted to the practice of thisinvention in its various embodiments and equivalents thereof.

EXAMPLES Example 1 Reaction Procedures

Girard's Reagent T/Cortisone Hydrazone Formation

Reactions were performed by adding 2 mL of a Girard's reagent T, (1 mMin 80:19.5:0.5 (v/v/v), methanol:water: formic acid) solution to 2 mL ofa cortisone (1 mM 80:20 (v/v), methanol:water) solution (Scheme 1). Halfof this sample was used for the Leidenfrost experiment while the otherhalf was used for the comparative bulk-phase experiment so that thesolutions have the same starting concentrations. Samples prepared at 4×concentrations were prepared in the same manner, but to 4 mM rather than1 mM concentrations.

Additional acid experiments were performed using the same reagentspreviously mentioned with 1:10 formic acid in methanol in a 1:1:1 ratiowith the other two reagents. 4× concentration experiments in this caseused the same amount of acid and simply a greater concentration of thereagents.

Phenylhydrazine/Isatin Hydrazone Formation

Combine 4 mL of 3 mM methanolic isatin with 40 uL phenylhydrazine and 4uL 1 M HCl (methanol) (Scheme 2). For the 4× concentration solutions allreagent concentrations were increased by a factor of four.

Katritzky Reaction

To 3 mL of 2,4,6-triphenylpyrilium (5 mM in acetonitrile) was added 1.9mL of 4-anisidine (8 mM in acetonitrile) (Scheme 3). This reaction wasalso performed with base in the same way but with the addition of 20 uLof 1.8 M methanolic potassium hydroxide. Bulk-phase reactions wereperformed at these concentrations and 4× concentrations. 4×concentration data was obtained using 20 mM 2,4,6-triphenylpyrilium, 32mM 4-anisidine, and by adding 80 uL of 1.8 M methanolic potassiumhydroxide.

Claisen-Schmidt Base-Catalyzed Condensation

This reaction was performed by mixing 2 mL 50 mM benzaldehyde (inmethanol), 2 mL 50 mM 6-hydroxy-1-indanone (in methanol) and 2 mL 1.8 mMmethanolic potassium hydroxide (Scheme 4). Reactions at 4× concentrationwere performed using 4× the concentration of the reagents and 4× thebase.

Triton experiments with this reaction were performed by mixing thetriton with the 6-hydroxy-1-indanone and then adding the aldehyde andbase.

Example 2 Determination of Acceleration Factor

Acceleration factors are used throughout this application and arecalculated using ratios of reaction product/starting material for theLeidenfrost conditions vs. bulk. Since the Leidenfrost droplets arethought to have the same or greater acceleration compared to thebulk-phase solution we have chosen to represent the factor as the ratioof Leidenfrost droplets ratio at a given time (t=2 mins) over the ratioof product/starting material of the bulk-phase reaction at the sametime. This ratio of ratios adjusts for the inherent variations inionization efficiencies for some of these reactions.

The approximations are extremely conservative estimates of theacceleration factor. More significant values could be attained in ratioof times would be used by comparing the 2 mins it takes to create asignificant product yield and correlate that to the amount of time ittakes to create the same product yield in the bulk-phase. In some cases,similar acceleration factors are achieved and in others significantlygreater acceleration factors can be seen. For example, the firstreaction explored, the reaction of Girard T with cortisone can take 2hours to create product but a similar amount of product can be createdin 2 minutes via the Leidenfrost experiments.

Example 3 nESI-MS Analysis

All mass analyses were performed using nanoESI-MS. The distance betweenthe tip of the spray emitter and ion transfer capillary to the MS washeld constant at ca. 1 mm. Experiments were performed using borosilicateglass pulled to a ca. 5 um opening. A spray voltage of either positiveor negative 1.5 kV was used for all analysis. All samples were dilutedat least 100-fold prior to mass analysis with either acetonitrile ormethanol. Positive ion mode was used for all chemical analysis exceptfor analysis for the Claisen-Schmidt base-catalyzed condensation sampleswhich used negative ion mode.

Example 4 Girard T/Cortisone Reaction Spectra

Acceleration factors measuring the product/reagent ratios with the 5 mMbulk concentrations would be 17; however, the concentration increase inthe Leidenfrost droplets due to evaporation must be taken into account.This was done by measuring the bulk reaction data measured at four timesthe initial concentration. Hydrazone formation from the Leidenfrostdroplets then shows a product (3)/reagent (1) ion ratio which is notablyhigher (factor of 4) than that for the bulk solution (FIGS. 5A-D).

Example 5 Scale-Up

FIG. 6 shows photographs of a multiplexing approach where the separatedLeidenfrost droplets are continually replenished with reaction mixture(left) and when the reaction had run for ca. 2 mins and all but two ofthe droplet spacers have been removed allowing the droplets to fuseprior to collection (right).

Example 6 Effect Concentration and Leidenfrost Effect

Exploring the effect of concentration on both bulk-phase reactions andreactions performed in the Leidenfrost experiments was important todecoupling the sources of acceleration (FIG. 6). For simplicity allreactions were performed for two minutes. Considering the case of theKatritsky reaction, the enhancement in bulk due simply to an increase inconcentration of 4× was found to be a factor of two (0.6/0.3), FIGS.7A-B. Simply adding solvent to the Leidenfrost droplet and maintainingconstant Leidenfrost volume without adding reaction mixture provided anacceleration of 4 (over the low concentration 2 min bulk), which can befully attributed to the Leidenfrost effect. Adding reaction mixturerather than solvent saw an acceleration factor of 17 over the 4×concentrated sample and 33 over the simple bulk in FIG. 6 (right panel).These findings and the surfactant findings discussed above illustratethat the roles of both concentration and surface effects are importantin causing reaction acceleration in the Leidenfrost effect. This issimilar to previous findings on reaction acceleration in ESI.

What is claimed is:
 1. A method for forming and collecting a reactionproduct, the method comprising: conducting a reaction within aLeidenfrost-levitated droplet while maintaining a substantially constantvolume of the Leidenfrost-levitated droplet, thereby forming a reactionproduct within the Leidenfrost-levitated droplet, wherein maintainingthe substantially constant volume comprises introducing droplets of puresolvent or the reaction mixture to the Leidenfrost-levitated droplet anda rate at which the droplets of the pure solvent or reaction mixture areintroduced to the Leidenfrost-levitated droplet is dependent on anevaporation rate of the Leidenfrost-levitated droplet; and collectingthe reaction product in the Leidenfrost-levitated droplet withoutevaporating the Leidenfrost-levitated droplet.
 2. The method accordingto claim 1, further comprising analyzing the reaction product.
 3. Themethod according to claim 2, wherein analyzing is by a mass spectrometrytechnique.
 4. The method according to claim 1, wherein the methodfurther comprises conducting multiple reactions in multiple separateLeidenfrost-levitated droplets.
 5. The method according to claim 4,wherein the multiple separate Leidenfrost-levitated droplets are mergedwith each other.
 6. The method according to claim 4, wherein themultiple reactions are the same.
 7. The method according to claim 4,wherein the multiple reactions are different.
 8. The method according toclaim 1, wherein the method is conducted without the use of surfactants.9. A method for forming and collecting a reaction product, the methodcomprising: introducing a first liquid droplet onto a heated surfacethat is at a temperate that is at or above a Leidenfrost point of thefirst liquid such that the first liquid droplet levitates on the heatedsurface, wherein the first liquid droplet comprises reagents for areaction; conducting a reaction in the first liquid droplet using thereagents in order to form a reaction product; introducing one or moresubsequent liquid droplets to the first liquid droplet on the heatedsurface such that the one or more subsequent liquid droplets merge withthe first liquid droplet, wherein a rate at which the one or moresubsequent liquid droplets are introduced to the first liquid droplet isdependent on an evaporation rate of the first liquid droplet; andcollecting the reaction product in the first liquid droplet withoutevaporating the first liquid droplet.
 10. The method according to claim9, further comprising analyzing the reaction product.
 11. The methodaccording to claim 10, wherein analyzing is by a mass spectrometrytechnique.
 12. The method according to claim 9, wherein the first liquiddroplet is maintained at a substantially constant volume throughout themethod.
 13. The method according to claim 9, wherein the one or moresubsequent liquid droplets each comprise the reagents for the reaction.14. The method according to claim 9, wherein the method is conductedwithout the use of surfactants.
 15. A method for forming and collectinga reaction product, the method comprising: conducting a reaction betweenat least two reagents within a Leidenfrost-levitated droplet, in orderto form a reaction product within the Leidenfrost-levitated droplet,wherein the method further comprises introducing one or more subsequentliquid droplets to the Leidenfrost-levitated droplet, wherein a rate atwhich the one or more subsequent liquid droplets are introduced to theLeidenfrost-levitated droplet is dependent on an evaporation rate of theLeidenfrost-levitated droplet; and collecting the reaction product inthe Leidenfrost-levitated droplet without evaporating theLeidenfrost-levitated droplet.
 16. The method according to claim 15,wherein the droplet reduces in volume as the reaction occurs.
 17. Themethod according to claim 15, wherein a rate of the reaction isaccelerated as compared to a rate of the reaction conducted outside ofthe Leidenfrost-levitated droplet.